One level of indentation per method

board-after.java

class Board 
{
  ...

  String board() 
  {
    StringBuffer buf = new StringBuffer();
    collectRows(buf);
    return buf.toString();
  }

  void collectRows(StringBuffer buf) 
  {
    for (int i = 0; i < 10; i++) 
    {
      collectRow(buf, i);
    }
  }

  void collectRow(StringBuffer buf, int row) 
  {
    for (int i = 0; i < 10; i++) 
    {
      buf.append(data[row][i]);
    }
    buf.append("\n");
  }
}